This project describes research accomplished on E. coli mutator genes during the FY1980-1981. Using transposons new alleles of mutD have been isolated and tested for complementation with each other. All alleles are dominant and non-complementing, suggesting that mutD is a regulatory gene controlling editing functions. An in vitro assay using 2-amino purine as an adenine analogue has been developed with crude E. coli extracts. Three different levels of error rate have been defined. One of these is altered in a mutator extract.